A manometer is a pressure-measuring instrument which is used to measure pressures either above or below atmospheric pressure. In either case the unknown pressure is estimated by noting the height of a liquid column that it will support. In its simplest form, the manometer consists of a U-tube as shown in FIG. 1, one end of which is open to the atmosphere, and the other end of which is coupled to the source of pressure to be measured. If the measured pressure is different from atmospheric pressure, the fluid with which the manometer tube is partially filled, usually mercury, will stand higher in one leg of the tube than the other.
There are several problems associated with the use of an open-end manometer. First, the manometer tube must provide at least 76 centimeters (39 inches) of displacement of mercury to support a pressure of one atmosphere. As a consequence, open-end manometers configured to measure even modest pressures are bulky devices. Furthermore, open-end manometers must remain stable and upright. If the manometer is rotated about its horizontal axis, or accidentally over-pressured, the manometer fluid will run out. This is especially undesirable when the fluid is mercury, which is toxic. Since scale calibration is also affected by angular orientation, it is generally the practice to mount U-tube manometers with the arms in a fixed position in order to prevent movement which would destroy the scale calibration. Therefore, open-end manometers are not readily adaptable to operate in various orientations and are practically limited to being used in a fixed location and position such as in a laboratory or shop.
A closed-end U-tube manometer is depicted in FIG. 2. This manometer is adaptable to a broader pressure range than open-end devices; however, as is the case with open-end manometers, closed-end manometers can lose their manometric fluid if rotated about a horizontal axis. Also, they are prone to loss of manometric fluid or loss of calibration if subjected to accidental pressure excursions. Furthermore, due to their relatively large size, and U-tube construction, it is essential that they remain stable and upright and therefore, they are essentially limited to stationary service.
The conventional mercury barometer is a special case of the straight tube, closed-end manometer. This device is useful only for indicating the pressure of the atmosphere at the open end of the tube, in absolute pressure units. The space between the end of the mercury column and the sealed end of the tube is evacuated and remains at zero pressure no matter what position the mercury column takes up.
It would be desirable, therefore, to have a manometer which is useful over a wide pressure range both above and below atmospheric pressure, and which is small in size, inexpensive enough to use on a one-time basis, and usable in any orientation.